Photovoltaic cell

ABSTRACT

The invention relates to a photovoltaic cell (1) comprising a substrate (2) having a support face (4) having a first electrode (6) thereon and a second electrode (10) spaced from the first electrode (6) by a plurality of layers (14, 16; 14, 24, 16) including at least one layer (14) of a semiconducting material with an active junction (J) interface thereat, said active junction (J) having a developed surface area greater than its projected surface area.

FIELD OF THE INVENTION

The invention relates to a photovoltaic cell and more specifically to acell of this type comprising a junction exhibiting a high roughnessfactor.

BACKGROUND OF THE INVENTION

The transformation of light energy into electrical energy usingphotovoltaic cells has been known for a long time and these cells arecurrently used in various electronic devices such as watches,calculators, cameras and the like.

These cells can be divided into four main types, namelymetal-semiconductor (MS) junction cells of the Schottky diode type,metal-insulator-semiconductor (MIS) junction cells,semiconductor-insulator-semiconductor (SIS) junction cells, andhomojunction or heterojunction cells.

The term junction is understood to mean the transition zone between ametal and a semiconductor or between two semiconductors of differenttypes of conductivity.

In all known thin layer photovoltaic cells the semiconducting materialused is always deposited in the form of a thin continuous and smoothlayer on the surface of a substrate previously covered by a firsttransparent electrode made, for example, of metal. This layer ofsemiconducting material is then covered, depending on the type of cell,by one or more layers (semiconducting and/or insulating and/orconducting), the upper most layer forming the second electrode.

Although developments and improvements have been made to thesephotovoltaic cells over the last few years to improve their efficiency,this efficiency still remains relatively modest.

Moreover, the manufacture of these cells requires the use of materialsof great purity and thus the use of sophisticated equipment installed inclean rooms, so that these cells still remain difficult to produce.

OBJECTS AND SUMMARY OF THE INVENTION

It is thus a main object of the invention to overcome the disadvantagesof the above-mentioned prior art by providing a photovoltaic cellexhibiting a high rate of conversion of the energy of incident lightinto electrical energy per unit of surface and which is, moreover,simple to manufacture.

The object of the invention is therefore a photovoltaic cell comprisinga substrate having a support face on which there is disposed a firstelectrode, a second electrode insulated from the first electrode by aplurality of layers having at least a first layer of a semiconductingmaterial with an active junction at an interface thereof, characterisedin that said active junction exhibits a developed surface area greaterthan its projected surface area.

This characteristic greatly increases the efficiency of collection ofincident photons in relation to the cells of the prior art. Thisincrease is essentially due to the multiple diffusion of the light inthe semiconducting layer in association with the large active junction.The structure of the cell of the invention thus provides an improvedefficiency per unit of surface area.

According to an advantageous feature of the invention, said activejunction exhibits a roughness factor greater than 20.

It will be noted in this connection that the roughness factor is definedby the ratio between the real surface area and the projected surfacearea.

According to a first embodiment of the invention, said support faceexhibits a developed surface area greater than its projected surfacearea and all said other layers extend successively on said support face.

This embodiment notably has the advantage of being simple since it onlyneeds one mechanical or chemical treatment of the substrate.

According to a second embodiment of the invention, the first electrodeexhibits a developed surface area greater than its projected surfacearea and all said other layers extend successively on said electrode.

According to a third embodiment of the invention, said first layer ofsemiconducting material exhibits a developed surface area greater thanits projected surface area and all said other layers extend successivelyon said first layer of semiconducting material.

According to another advantageous feature of the invention common to thesecond and to the third embodiment, said first electrode or the firstlayer of semiconducting material comprises a layer formed of colloidalparticles.

This feature endows the active junction with a very high effectivesurface and thus also a very high effective surface area/projectedsurface area ratio which can reach a value of the order of 2000.

This layer also enables the minority carriers produced within it by theincident photons to reach the junction before any recombination of thesecarriers occurs and consequently to produce a photovoltaic cell whichmakes the best use of the energy supplied by the photons from theincident light.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will emerge from study ofthe description of embodiments of the photovoltaic cell of the inventiongiven by way of non-limiting illustration in connection with theappended drawings in which:

FIG. 1 represents diagrammatically and in section a first type ofphotovoltaic cell according to the invention;

FIGS. 2 to 6 are, respectively, partially enlarged views of the cell ofFIG. 1 according to various embodiments of the invention;

FIG. 7 shows diagrammatically and in section a second type ofphotovoltaic cell according to the invention; and

FIGS. 8 to 12 are respectively partially enlarged views of the cell ofFIG. 7 according to various embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in an application tometal-semiconductor junction photovoltaic cells of the Schottky type(MS) or heterojunction or homojunction semiconductor-semiconductorphotovoltaic cells in connection with FIGS. 1 to 6 and then in anapplication to metal-insulator-insulator-semiconductor (MIS) junctionphotovoltaic cells or semiconductor-insulator-semiconductor (SIS)photovoltaic cells in connection with FIGS. 7 to 12.

The principle of operation of these large families of photovoltaic cellsis well known to the person skilled in the art and reference willtherefore only be made thereto in the following description where thereis a connection between this principle and the invention. Reference maynotably be made to the work entitled "Microelectronic Devices" by EdwardS. Yang for an explanation of the physical phenomena used inphotovoltaic cells.

Referring first of all to FIG. 1, this shows a first type ofphotovoltaic cell according to the invention designated by the generalreference numeral 1.

The photovoltaic cell 1 comprises a substrate 2 having on the entiresurface of one of its major faces 4, termed a support face, a firstelectrode 6 connected to a first conductor 8. The cell 1 comprises asecond electrode 10 connected to a second conductor 12, separated fromthe first electrode 6 by a first layer 14 of a semiconducting materialand a layer 16 of a different material. Depending on the type ofphotovoltaic cell envisaged and as will emerge in the followingdescription, the material of this layer 16 can either be an electronicconductor (MS cell) or a semiconductor (homojunction or heterojunctiontype cell). This layer will hereinafter be referred to as a conductinglayer or second layer of semiconducting material, depending on the typeof cell being described.

The term electronic conducting layer naturally also refers to a layermade of organic electronic conducting material and the termsemiconducting materials refers both to inorganic semiconductingmaterials and to organic semiconductors.

The first layer 14 of semiconducting material is in direct contact withthe first electrode 6 and together with the layer 16 produces an activejunction designated J in FIGS. 2 to 5 and 8 to 10.

The first electrode 6 is preferably made in the form of a thin layer ofthe order of 10 to 500 nanometers in thickness. The electrode 6 isadvantageously made of a material chosen from the group consisting oftin oxide doped with fluorine, antimony or arsenic, indium oxide dopedwith tin oxide, aluminium stannate, and zinc oxide doped with aluminium.

The person skilled in the art may of course choose any other equivalenttransparent electronic conducting layer.

The nature of the second electrode 10 depends on the nature of the layer16 and can be omitted if the latter is electrically conducting. If thelayer 16 is not conducting, the second electrode 10 should preferably bemade in the form of a thin layer of a material such as gold or aluminiumor of a material exhibiting analogous electrical conductivityproperties.

Either the substrate 2 and the first electrode or the second electrode10 are of course transparent to photons of the relevant spectral range.

It will be noted that the figure does not reflect the exact dimensionsof the cell formed in this manner, these dimensions having been greatlyexaggerated for reasons of clarity.

According to the invention, the active junction J exhibits a developedsurface area greater than its projected surface area.

In a first embodiment of the invention, the support face 4 of thesubstrate 2 exhibits a developed surface area greater than its projectedsurface area. The following layers which extend successively on thesubstrate 2 closely embrace the configuration in relief of the supportface 4 in such a way that the texture of the face 4 of the substrate 2results in the active junction J having a developed surface area greaterthan its projected surface area.

Care should of course also be taken to ensure that the layer forming theelectrode 6 as well as the first layer 14 are of a thickness such thatthe developed surface area of the active junction J is substantiallyequal to that of the support face 4 of the substrate.

The support face 4 preferably exhibits a roughness factor greater than20 and typically a roughness factor of the order of 100, making itpossible to obtain a cell having a relatively large efficacy of lightcollection by multiple diffusion of the light in relation to the cellsof the prior art.

The appropriate roughness factor of this support face 4 can be obtainedsimply, for example by abrasion or also by chemical attack. If thesubstrate 2 is made of an organic material, the roughness factor of thesupport face may, for example, be obtained by moulding.

The other layers are successively deposited in a conventional manner forexample by vapor phase chemical deposition or by physical deposition ina vacuum.

According to a second embodiment of the invention shown in FIG. 3, andin contradistinction to the first embodiment, the support surface 4 issmooth and it is the face of the first electrode 6 opposite this face 4which has a rough appearance. The other layers, in particular the firstlayer 14 of semiconducting material, extend successively on the firstelectrode 6 and closely embrace its relief. Here, too, the roughnessfactor is advantageously greater than 20 and preferably of the order of100.

To obtain this roughness factor, the first electrode 6 can for examplebe deposited by tangential evaporation in a vacuum (the substrate beinginclined by 3° to 20° in relation to the direction of evaporation), theother layers being conventionally deposited as has already beenindicated hereinabove.

According to a third embodiment, shown in FIG. 4, the support face 4 ofthe substrate 2 and the first electrode 6 do not exhibit a specialroughness factor, but the face of the first layer 14 of semiconductingmaterial which is in contact with the layer 16, that is the activejunction J, is rough and advantageously exhibits a roughness factorgreater than 20 and preferably of the order of 100.

According to this embodiment, the first layer 14 of semiconductingmaterial can, for example, be deposited by tangential evaporation in avacuum.

Reference now being made to FIG. 5, this shows another embodiment of thecell of the invention in which the first electrode 6 comprises a uniformlayer 18 formed of a conducting material, such as those describedhereinabove, on which a layer of colloidal particles 20 of the samematerial is, for example, deposited by a sol-gel process. The followinglayers are successively deposited on the electrode 6 as describedhereinabove.

The layer 18 typically exhibits a thickness within a range extendingfrom 15 to 150 nanometers and the colloidal particles 20 preferablyexhibit a diameter of 1 to 200 nanometers. These particles areagglomerated and form a porous layer the thickness of which is between0.1 and 20 micrometers at the surface of the substrates.

This therefore makes it possible to produce a first layer 14 ofsemiconducting material and thus an active junction exhibiting an activesurface area/projected surface area ratio of the order of 2000 andconsequently a cell having a very high active surface.

For this embodiment to provide maximum efficiency, care must be taken toensure that the first layer 14 of semiconducting material has athickness lower than or equal to half the width of the pores 22 formedby the particles 20, otherwise the pores 22 become blocked, therebyconsiderably reducing the roughness factor of the active junction J.

According to a fifth embodiment, shown in FIG. 6, the support face 4 ofthe substrate 2 and the first electrode 6 do not exhibit any specialroughness, but the first layer 14 of semiconducting material comprises alayer formed of colloidal particles 20 on which the following layers aredeposited.

In this case, the size of the particles 20 and the thickness of thelayer formed by the particles have a considerable effect on the responseof the cell to excitation by incident light.

The particles of the layer absorb the photons, the energy of which isequal to or exceeds the difference in energy between the conduction bandand the valence band of the material of the particles. This absorptionof light leads to the creation of electron-hole pairs in the particles.With, for example, a layer 14 made of a semiconducting material such asn type TiO₂, the electrons are the majority carriers whereas the holesare the minority carriers. Thus, when a semiconductor/metal junctioncell, or a semiconductor/semiconductor junction cell of this type isused to create electricity from absorbed light, it is necessary for theholes to be able to diffuse up to this junction before they can combinewith the electrons. In other words, the diffusion length of the minoritycarriers, designated by 1_(pm), must be larger than the distance whichthese carriers have to travel before reaching the junction.

This diffusion length is defined by

    1.sub.pm =(2Dτ).sup.0.5

in which τ is the life time of a hole and D is the diffusion constant ofthe minority carriers. By way of example, the value of 1_(pm) is 100nanometers for TiO₂.

The diameter of the colloidal particles 20 must thus preferably be lessthan the diffusion length of the minority carriers for there to be agreat probability that these carriers will reach the semiconductor/metalor semiconductor/semiconductor junction and to thereby achieve aneffective separation of the charge carriers and a high efficiency ofconversion is increased.

Reference now being made to FIGS. 7 to 12, these show variousembodiments of metal-insulator-semiconductor (MIS)semiconductor-insulator-semiconductor (SIS) junction photovoltaic cellsaccording to the invention, in which the elements identical to thosedescribed in connection with FIGS. 1 to 6 are designated by the samereference numerals.

As may be clearly seen in FIGS. 7 to 12, the structure of thesephotovoltaic cells does not differ from the cells described inconnection with FIGS. 1 to 6 except that one layer 24 of an insulatingmaterial extends between the first layer 14 of semiconducting materialand the layers 16 and 10 which are thereabove, regardless of the natureof these layers 16 and 10.

The layer 24 of insulating material advantageously exhibits a thicknessbetween 1 and 50 nanometers. In any case, the thickness of this layermust permit injection by the tunnel effect of electrons of the firstlayer 14 of semiconducting material into the layer 16 forming the secondlayer of semiconducting material, in the case of an SIS cell or into themetal layer in the case of an MIS cell.

The first layer 14 of semiconducting material can be made of aninorganic semiconducting material. The same applies to the layer 16 inthe case of an SIS cell.

Nevertheless, in the case of the embodiment of the cell according to theinvention shown in FIG. 11, care should be taken, as shown in FIG. 5,that the first layer 14 of semiconducting material has a thickness lessthan or equal to half the dimension of the pores 22 formed by theparticles 20, otherwise these pores 22 become blocked, therebyconsiderably reducing the roughness factor.

The semiconducting material constituting the layers 14 (MS, MIS, SIS,homojunction and heterojunction cells) and/or 16 (SIS, homojunction andheterojunction cells) which have just been described can be chosen fromamongst the semiconducting materials included in the following fourgroups:

The first group consists of oxides of the transition elements, oxides ofthe elements of columns 13 and 14 of the modern periodic classificationand lanthanide oxides (see Cours de chimie physique by Paul Arnaud,published by Dunod 1988).

The second group consists of mixed oxides formed of a mixture of two ormore oxides of the first group.

The third group consists of mixed oxides formed of a mixture of one ormore oxides of the first group with oxides of the elements of columns 1and 2 of the modern periodic classification.

The fourth group is formed of the group of semiconducting materialsconsisting of:

silicon, silicon hydride, silicon carbide, germanium, cadmium sulphide,cadmium telluride, zinc sulphide, lead sulphide, iron sulphide, zinc andcadmium sulphide, zinc selenide,gallium arsenide, indium phosphide,gallium phosphide, cadmium phosphide, titanium fluoride, titaniumnitride, zirconium fluoride, zirconium nitride, doped diamond, copperthiocyanate, and pure and mixed chalcopyrites.

The semiconducting material is preferably chosen from the group ofmaterials consisting of titanium oxide, lanthanum oxide, zirconiumoxide, niobium oxide, tungsten oxide, strontium oxide, calcium/titaniumoxide, sodium titanate, and potassium niobate.

According to a particular embodiment of the invention, the layers 14and/or 16 can be made of an organic semiconducting material.

The organic semiconducting material constituting these layers 14 and/or16 can be chosen from the semiconducting materials of the groupconsisting of phthalocyanines (hereinafter designated Pc), 2,9-dimethylquinacridone, 1,1-bis (4-di-p-polylaminophenyl) cyclohexane,phthalocyanine bisnaphthalocyanine, poly(N-vinylcarbazole),polyanthracenes, polyphenols, polysilanes, poly(p-phenylene) vinylene,porphyrins, perylene and its derivatives,poly(benzo[C]thiophene)=poly(isothianaphthene), polythiophene,poly(3-methylthiophene), poly(3-octylthiophene), polyaniline,poly(p-phenylene), poly(thiophene)vinylene, polyacetylene, polyazulene,and diacetylenes.

The organic semiconducting material is preferably chosen from the groupof materials consisting of H₂ Pc, MgPc doped with O₂, CuPc, ZnPc, FePc,SiPc, NiPc, Al(Cl)Pc, Al(OH)Pc, LuPc₂ doped with dichlorocyanoquinone,tetra-4-terbutylphthalocyanino silicon dichloride,LuPc2:2,2'6,6'-tetraphenyl-4-4'((p-dimethylaminostyryl)-4H-pyrane and5,10,15,20-tetra(3-pyridyl) porphyrine, LuPc, and NiPc:I₂.

Of course, the semiconducting material can have an n or p conductivitytype depending on the type of cell proposed.

The conducting layer 16 in the case of MS or MIS cells can be made of amaterial chosen from the group of metals consisting of platinum,ruthenium, rhodium, palladium, iridium, silver, osmium, gold, platinum,aluminum, indium, magnesium, and conducting oxides of the elements ofcolumns 8 to 10 of the modern periodic classification.

According to a variant, the conducting layer 16 of MS or MIS cells canbe formed by a conducting polymer advantageously selected from the groupconsisting of poly(benzo[C]thiophene), poly(isothianaphthene),polythiophene, poly(3-methylthiophene), poly(3-octylthiophene),polyaniline, poly(p-phenylene), poly(thiophene)vinylene, polyacetylene,polyazulene, diacetylenes, and doped and undoped phthalocyanines.

In the metal-insulator-semiconductor (MIS) orsemiconductor-insulator-semiconductor (SIS) photovoltaic junction cellsof the invention, the layer of insulating material 24 can be made of amaterial chosen from a first group consisting of aluminum oxide, siliconoxide, zirconium oxide, yttrium oxide, lanthanum oxide, aluminumoxyfluoride, cubic boron nitride, diamond, a second group of metaloxides having a forbidden band greater than 3.5 eV, and a third group ofinsulating polymers consisting of polyimide, polymetamethylacrylate,polyethylene, polypropylene, polystyrene, and polysilanes.

It will be noted that, according to the invention, it is possible toobtain a transparent or almost transparent photovoltaic cell by means ofjudicious selection of the various materials used to produce the cell.

We claim:
 1. A photovoltaic cell comprising a substrate having a supportface having disposed thereon a first electrode and a second electrodeseparated from the first electrode by a plurality of layers comprisingat least a first layer of a semiconducting material with an activejunction at an interface thereof, said active junction having adeveloped surface area greater than its projected surface area, saidfirst electrode comprising a uniform layer formed of a conductingmaterial and a porous layer formed of conducting colloidal particles,which has a developed surface area greater than its projected surfacearea and on which the other layers and said second electrode aredisposed successively, and said first layer of semiconducting materialhaving a thickness less than or equal to half the width of the pores ofsaid porous layer.
 2. A cell according to claim 1, wherein said activejunction exhibits a roughness factor greater than
 20. 3. A cellaccording to claim 1, wherein said conducting colloidal particles have adiameter between 1 and 200 nanometers.
 4. A cell according to claim 1,wherein said layer formed of conducting colloidal particles has athickness between 0.1 and 20 micrometers.
 5. A cell according to claim1, wherein said plurality of layers comprises, apart from said firstlayer of semiconducting material, a conducting layer which extendsbetween said first layer of semiconducting material and said secondelectrode.
 6. A cell according to claim 5, wherein said plurality oflayers also comprises a layer of insulating material extending betweensaid first layer of semiconducting material and said conducting layer.7. A cell according to claim 6, wherein the layer of insulating materialis made of a material selected from a first group consisting of aluminumoxide, silicon oxide, zirconium oxide, yttrium oxide, lanthanum oxide,aluminum oxyfluoride, cubic boron nitride, diamond, a second groupconsisting of metal oxides having a forbidden band greater than 3.5 eV,and a third group consisting of polyimide, polymetamethylacrylate,polyethylene, polypropylene, polystyrene, and polysilanes.
 8. A cellaccording to claim 5, wherein said second electrode is formed by saidconducting layer.
 9. A cell according to claim 5, wherein the conductinglayer is made of a material selected from a first group consisting ofpoly(benzo[C]thiopene), poly(isothianaphthene), polythiophene,poly(3-methylthiophene), poly(3-octylthiophene), polyaniline,poly(p-phenylene), poly(thiophene)vinylene, polyacetylene, polyazulene,diacetylenes, and doped or undoped phthalocyanines and a second groupconsisting of platinum, ruthenium, rhodium, palladium, iridium, osmium,silver, gold, platinum, aluminum, indium, magnesium, and conductingoxides of the elements of columns 8 to 10 of the modern periodicclassification.
 10. A cell according to claim 1, wherein said pluralityof layers comprises, apart from said first layer of semiconductingmaterial, a second layer of semiconducting material of a different typeof conductivity from that of the first layer of semiconducting materialand which extends between said first layer and the second electrode. 11.A cell according to claim 10, wherein said plurality of layers alsocomprises a layer of insulating material extending between said firstlayer of semiconducting material and said second layer of semiconductingmaterial.
 12. A cell according to claim 10, wherein said first layer ofa semiconducting material and/or said second semiconducting layer is(are) made of an inorganic semiconducting material.
 13. A cell accordingto claim 10, wherein said first layer of semiconducting material and/orsaid second layer of semiconducting material is (are) made of an organicsemiconducting material.
 14. A cell according to claim 13, wherein saidfirst layer of semiconducting material and/or said second layer ofsemiconducting material is (are) made of a semiconducting materialselected from the group of doped and/or undoped semiconducting materialsconsisting of phthalocyanines, 2,9-dimethyl quinacridone, 11-bis(4-di-p-tolylaminophenyl) cyclohexane, phthalocyaninebisnaphthalocyanine, poly (N-vinylcarbazole) , polyanthracenes,polyphenols, polysilanes, poly (p-phenylene) vinylene, porphyrins,perylene and its derivatives, poly(benzo[C]thiophene,poly(isothianaphthene), polythiophene, poly(3-methylthiophene),poly(3-octylthiophene), polyaniline, poly(p-phenylene),poly(thiophene)vinylene, polyacetylene, polyazulene, and diacetylenes.15. A cell according to claim 13, wherein said first layer ofsemiconducting material and/or said second layer of semiconductingmaterial is (are) made of a semiconducting material selected from thegroup of semiconducting materials consisting of H₂ Pc, MgPc doped withO₂, CuPc, ZnPc, FePc, SiPc, NiPc, Al(Cl)Pc, Al (OH)Pc, LuPc₂ doped withdichlorocyanoquinone, tetra-4-tert-butylphthalocyanino silicondichloride, LuPc:2,2'6,6'-tetraphenyl-4,4'(p-dimethylaminostyryl)4H-pyrane and 5,10,15,20-tetra (3-pyridyl)porphyrin, LuPc, and NiPc: I₂.16. A cell according to claim 10, wherein said first layer ofsemiconducting material and/or said second layer of semiconductingmaterial is (are) made of a semiconducting material selected fromsemiconducting materials formed of a first group consisting ofsemiconducting oxides of the transition elements, semiconducting oxidesof the elements of columns 13 and 14 of the modern periodicclassification and semiconducting lanthanide oxides, a second groupconsisting of mixed semiconducting oxides formed of a mixture of two ormore oxides of the first group, a third group consisting of mixedsemiconducting oxides formed of a mixture of one or more oxides of thefirst group with oxides of the elements of columns 1 and 2 of the modernperiodic classification, and a fourth group consisting of silicon,silicon hydride, silicon carbide, germanium, cadmium sulphide, cadmiumtelluride, zinc sulphide, lead sulphide, iron sulphide, zinc selenide,gallium arsenide, indium phosphide, gallium phosphide, cadmiumphosphide, titanium fluoride, titanium nitride, zirconium fluoride,zirconium nitride, doped diamond, copper thiocyanate, and pure and mixedchalcopyrites.
 17. A cell according to claim 16, wherein thesemiconducting material is selected from the group of materialsconsisting of titanium oxide, lanthanum oxide, zirconium oxide, niobiumoxide, tungsten oxide, strontium oxide, calcium/titanium oxide, sodiumtitanate, and potassium niobate.
 18. A cell according to claim 1,wherein the first electrode is made of a material chosen from the groupconsisting of tin oxide doped with fluorine, antimony or arsenic,aluminum stannate, and zinc oxide doped with aluminum.
 19. A cellaccording to claim 1, wherein said first layer of a semiconductingmaterial is made of an inorganic semiconducting material.
 20. A cellaccording to claim 1, wherein said first layer of semiconductingmaterial is made of an organic semiconducting material.
 21. Aphotovoltaic cell comprising a substrate having a support face havingdisposed thereon a first electrode and a second electrode separated fromthe first electrode by a plurality of layers comprising at least a firstlayer of a semiconducting material with an active junction at aninterface thereof, said active junction having a developed surface areagreater than its projected surface area, said first layer ofsemiconducting material comprising a porous layer formed of colloidalparticles and having a developed surface area greater than its projectedsurface area and a uniform layer covering said porous layer and on whichthe other layers and said second electrode are disposed successively,and said colloidal particles having a diameter smaller than thediffusion length of minority charge carriers created in said porouslayer by the absorption of light.
 22. A cell according to claim 21,wherein said active junction exhibits a roughness factor greater than20.
 23. A cell according to claim 21, wherein said colloidal particleshave a diameter between 1 and 200 nanometers.
 24. A cell according toclaim 21, wherein said layer formed of colloidal particles has athickness between 0.1 and 20 micrometers.
 25. A cell according to claim21, wherein said plurality of layers comprises, apart from said firstlayer of semiconducting material, a conducting layer which extendsbetween said first layer of semiconducting material and said secondelectrode.
 26. A cell according to claim 25, wherein said plurality oflayers also comprises a layer of insulating material extending betweensaid first layer of semiconducting material and said conducting layer.27. A cell according to claim 26, wherein the layer of insulatingmaterial is made of a material selected from a first group consisting ofaluminum oxide, silicon oxide, zirconium oxide, yttrium oxide, lanthanumoxide, aluminum oxyfluoride, cubic boron nitride, diamond, a secondgroup consisting of metal oxides having a forbidden band greater than3.5 eV, and a third group consisting of polyimide,polymetamethylacrylate, polyethylene, polypropylene, polystyrene, andpolysilanes.
 28. A cell according to claim 25, wherein said secondelectrode is formed by said conducting layer.
 29. A cell according toclaim 25, wherein the conducting layer is made of a material selectedfrom a first group consisting of poly(benzo[[C]thiophene),poly(isothianaphthene), polythiophene, poly(3-methylthiophene),poly(3-octylthiophene), polyaniline, poly(p-phenylene),poly(thiophene)vinylene, polyacetylene, polyazulene, diacetylenes, anddoped or undoped phthalocyanines and a second group consisting ofplatinum, ruthenium, rhodium, palladium, iridium, osmium, silver, gold,platinum, aluminum, indium, magnesium, and conducting oxides of theelements of columns 8 to 10 of the modern periodic classification.
 30. Acell according to claim 21 wherein said plurality of layers comprises,apart from said first layer of semiconducting material, a second layerof semiconducting material of a different type of conductivity from thatof said first layer of semiconducting material and which extends betweensaid first layer and the second electrode.
 31. A cell according to claim30 wherein said plurality of layers also comprises a layer of insulatingmaterial extending between said first layer of semiconducting materialand said second layer of semiconducting material.
 32. A cell accordingto claim 30, wherein said first layer of a semiconducting materialand/or said second semiconducting layer is (are) made of an inorganicsemiconducting material.
 33. A cell according to claim 32, wherein saidfirst layer of semiconducting and/or said second layer of semiconductingmaterial is (are) made of a semiconducting material selected fromsemiconducting materials formed of a first group consisting ofsemiconducting oxides of the transition elements, semiconducting oxidesof the elements of columns 13 and 14 of the modern periodicclassification and semiconducting lanthanide oxides, a second groupconsisting of mixed semiconducting oxides formed of a mixture of two ormore oxides of the first group, a third group consisting of mixedsemiconducting oxides formed of a mixture of one or more oxides of thefirst group with oxides of the elements of columns 1 and 2 of the modernperiodic classification, and a fourth group consisting of silicon,silicon hydride, silicon carbide, germanium, cadmium sulphide, cadmiumtelluride, zinc sulphide, lead sulphide, iron sulphide, zinc selenide,gallium arsenide, indium phosphide, gallium phosphide, cadmiumphosphide, titanium fluoride, titanium nitride, zirconium fluoride,zirconium nitride, doped diamond, copper thiocyanate, and pure and mixedchalcopyrites.
 34. A cell according to claim 33, wherein thesemiconducting material is selected from the group of materialsconsisting of titanium oxide, lanthanum oxide, zirconium oxide, niobiumoxide, tungsten oxide, strontium oxide, calcium/titanium oxide, sodiumtitanate, and potassium niobate.
 35. A cell according to claim 30,wherein said first layer of semiconducting material and/or said secondlayer of semiconducting material is (are) made of an organicsemiconducting material.
 36. A cell according to claim 35, wherein saidfirst layer of semiconducting material and/or said second layer ofsemiconducting material is (are) made of a semiconducting materialselected from the group of doped and/or undoped semiconducting materialsconsisting of phthalocyanines, 2,9-dimethyl quinacridone,1,1-bis(4-di-p-tolylaminophenyl) cyclohexane, phthalocyaninebisnaphthalocyanine, poly(N-vinylcarbazole), polyanthracenes,polyphenols, polysilanes, poly(p-phenylene)vinylene, porphyrins,perylene and its derivatives, poly(benzo[C]thiophene),poly(isothianaphthene), polythiophene, poly(3-methylthiophene),poly(3-octylthiophene), polyaniline, poly(p-phenylene),poly(thiophene)vinylene, polyacetylene, polyazulene, and diacetylenes.37. A cell according to claim 35, wherein said first layer ofsemiconducting material and/or said second layer of semiconductingmaterial is (are) made of a semiconducting material selected from thegroup of semiconducting materials consisting of H₂ Pc, MgPc doped withO₂, CuPc, ZnPc, FePc, SiPc, NiPc, Al(Cl)pc, Al(OH)Pc, LuPc₂ doped withdichlorocyanoquinone, tetra-4-tert-butylphthalocyanino silicondichloride,LuPc:2,2'6,6'-tetraphenyl-4,4'(p-dimethylaminostyryl)4H-pyrane and5,10,15,20-tetra(3-pyridyl)porphyrin, LuPc, and NiPc:I₂.
 38. A cellaccording to claim 21, wherein the first electrode is made of materialchosen from the group consisting of tin oxide doped with fluorine,antimony or arsenic, aluminum stannate, and zinc oxide doped withaluminum.
 39. A cell according to claim 21, wherein said first layer ofa semiconducting material is made of an inorganic semiconductingmaterial.
 40. A cell according to claim 21, wherein said first layer ofsemiconducting material is made of an organic semiconducting material.